Organic, conductive polymers have become increasingly wide-spread in industry. Areas of application are, for example, the through-plating of circuit boards (EP 553 671 A1), the antistatic finishing of photographic films (EP 440 957 A2) or as electrode in solid-electrolyte capacitors (EP 340 512 A2). Particular importance has been achieved by poly-3,4-alkylenedioxythiophenes, which are distinguished by high stability and electrical conductivity.
EP 440 957 A2 describes water-soluble or water-dispersible polyalkylenedioxythiophenes. These are synthesized directly in the presence of polyanions in the aqueous phase. The aqueous preparations are employed for the production of conductive or antistatic coatings. For many applications, however, there is a need for polyalkylenedioxythiophenes which can be processed in the form of anhydrous or low-water-content solutions or dispersions, for example for shortening the drying times of the coatings or for improving the wetting behavior. After removal of the water from the solutions described in EP 440 957 A2, the solids obtained are insoluble in water and organic solvents.
EP 339 340 A2 describes the preparation of 3,4-polyalkylenedioxythiophenes by reaction of the corresponding monomers with oxidants. The 3,4-polyalkylenedioxythiophenes prepared in this way are likewise insoluble in organic and inorganic solvents.
EP 440 957 A2 teaches that the preparation of water-soluble or water-dispersible polyalkylenedioxythiophenes can in principle also be carried out in the presence of protic solvents, such as methanol, ethanol, isopropanol and other water-miscible organic solvents, such as, for example, acetone. However, only small amounts of organic solvents can be admixed here, and consequently anhydrous or low-water-content solutions or dispersions cannot be obtained in this way.
Polyalkylenedioxythiophenes are preferably prepared by oxidative polymerization. Known oxidants are transition metals, salt-like compounds and other oxidants. Preference is given here to the use of transition metals and salt-like oxidants, in particular alkali metal or ammonium peroxodisulfates.
Although there have been attempts to use transition metal-containing oxidants, such as iron(III) salts, for the polymerization of thiophenes in organic solvents, such as, for example, chloroform, acetonitrile and methanol, this is afflicted with difficulties and disadvantages. The polythiophenes prepared in this way are not or only partially dispersible in solvents or are only dispersible in solvents with the aid of further dispersion assistants (see in this respect, for example, Lee, S. Park and Y. Son, Mol. Cryst. Liq. Cryst. 327 (1999) 237–240). The transition metal-containing residues of the oxidants can only be removed from the reaction solutions with difficulty and have an adverse effect on the quality of the conductive or antistatic coatings, such as, for example, their ageing resistance.
Salt-like oxidants, such as peroxodisulfate or perborates, and other salt-like compounds only react very slowly if at all in the absence of water or in the presence of only small amounts of water, giving rise to unacceptably long reaction times when the reaction is carried out in low-water-content solvents. The reaction solutions obtainable in this way contain large amounts of monomers and virtually no polythiophenes and therefore cannot be used.
Surprisingly, it has now been found that polythiophenes of high quality that are readily soluble or dispersible in anhydrous or low-water-content solvents can be prepared if these are prepared in anhydrous or low-water-content solvents and phase-transfer catalysts are added during the reaction. The preparation process has the advantage that it is possible to prepare solvent-containing anhydrous or low-water-content polythiophene dispersions or solutions that have only low metal and salt contents after work-up. The polythiophenes prepared by the process according to the invention are stable on storage as a solid, dispersion or solution and are suitable for the production of conductive or antistatic layers of high quality.